System and method for reducing noise in sensors with capacitive pickup

ABSTRACT

An apparatus and method are provided for reducing noise in a capacitive sensor ( 200 ). One apparatus includes a gain stage ( 210 ) including an output, the gain stage configured to generate a first signal having a noise component and a second signal having a desired output component and the noise component, and a filtered-sampling stage ( 250 ) having an input coupled to the gain stage output, the filtered-sampling stage configured to sample the first signal, store the first signal, and subtract the first signal from the second signal to produce a desired output signal. A method includes generating a first signal having a first noise component of the gain stage ( 710 ), storing the first signal ( 725 ), generating a second signal comprising a desired output component and the first noise component ( 730 ), and subtracting the first signal from the second signal to produce a first output signal having the desired output component ( 750 ).

FIELD OF THE INVENTION

The present invention generally relates to sensors, and moreparticularly relates to reducing noise in capacitive sensors.

BACKGROUND OF THE INVENTION

Signals output by capacitive sensors often include noise from variouscomponents (e.g., noise from an operational amplifier, noise from one ormore switches, etc.) of the capacitive sensor. Often, the noisecomponent included in the output signals is large and represents a majorlimitation in achieving greater sensitivity in detecting changes incapacitance. Accordingly, it is desirable to provide systems and methodsfor generating output signals of a capacitive sensor with reducedamounts of noise. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description of the invention and the appendedclaims, taken in conjunction with the accompanying drawings and thisbackground of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a schematic diagram of a known capacitive element sensor 100having a gain stage coupled to a sampling stage;

FIG. 2 is a schematic diagram of one exemplary embodiment of acapacitive sensor having a gain stage and a passive filtered-samplingstage;

FIG. 3 is a schematic diagram of another exemplary embodiment of acapacitive sensor having a gain stage and a passive filtered-samplingstage;

FIG. 4 is a schematic diagram of yet another exemplary embodiment of acapacitive sensor having a gain stage and a passive filtered-samplingstage;

FIG. 5 is a schematic diagram of one exemplary embodiment of acapacitive sensor having a gain stage and an active filtered-samplingstage;

FIG. 6 is a schematic diagram of another exemplary embodiment of acapacitive sensor having a gain stage and an active filtered-samplingstage; and

FIG. 7. is a flow diagram representing one exemplary embodiment of amethod for reducing noise in a capacitive sensor.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

FIG. 1 is a diagram of a prior art capacitive element sensor 100including a gain stage 110 coupled to a stage 150. Gain stage 110typically includes a capacitive element 112 having a measurablecapacitance (C_(s)) and a capacitive element 114 having a referencecapacitance (C_(ref)). Capacitive elements 112 and 114 are eachselectively coupled to a reference voltage (V_(ref)) and ground viaswitches 116 and 118, respectively, such that only one of capacitiveelements 112 and 114 is coupled to V_(ref) and ground at any particulartime. That is, when switch 116 couples capacitive element 112 toV_(ref), switch 118 couples capacitive element 114 to ground. Similarly,when switch 118 couples capacitive element 114 to V_(ref), switch 116couples capacitive element 112 to ground.

Capacitive elements 112 and 114 are each coupled to an integratorcircuit comprised of an operational amplifier 120 having a positiveinput, a negative input, and an output, and a capacitive element 125having a capacitance (C₁₂₅) coupled to the negative input and to theoutput of operational amplifier 120. In addition, gain stage 110includes a switch 130 so that signals may discharge capacitive element125 when switch 130 is closed.

Stage 150 includes a switch 155 to selectively couple stage 150 to anoutput node 165 and a capacitive element 160. Capacitive element 160 iscoupled between node 165 and ground.

The purpose of capacitive element sensor 100 is to produce a voltageoutput proportional to the capacitive difference of capacitive elements112 and 114 (i.e., C_(s)−C_(ref)). The following is a description of howcapacitive sensor 100 operates:

Switches 130 and 155 are closed, switch 116 is connected to V_(ref), andswitch 118 is connected to ground. This results in the output signalbeing equal to V_(ref).

Switch 130 is opened, while switch 155 remains closed. This causes theinstantaneous noise of capacitive sensor 100 to be sampled and held atcapacitive element 125. Shortly after switch 130 is opened, switches 116and 118 are switched so that voltage steps of V_(ref) and −V_(ref) areapplied to capacitive elements 114 and 112, respectively. The change incharge (i.e., V_(ref)*(C_(s)−C_(ref))) is stored at capacitive element125, which leads to a change in the output signal by the desired amountV_(ref)*(C_(s)−C_(ref))/C₁₂₅. Switch 155 is then opened so that thepresent value of the output signal is stored on capacitive element 160.

The change in the output signal also includes noise generated byoperational amplifier 120, which noise is represented by a virtual noisesource 122, and noise generated by the switching action of switch 130.Furthermore, the output signal stored in capacitive element 160 alsoincludes noise generated by the switching action of switch 155. That is,while the ideal output signal is only V_(ref)*(C_(s)−C_(ref))/C₁₂₅,capacitive element 160 stores a signal comprised of several othercomponents represented by the following equation:V_(ref)*(C_(s)−C_(ref))/C₁₂₅ plus operational amplifier 120 noise, plusswitch 130 noise, plus switch 155 noise.

FIG. 2 is a schematic diagram of one exemplary embodiment of acapacitive sensor 200 having a gain stage 210 and a passivefiltered-sampling stage 250. Gain stage 210 may be any device,circuitry, hardware, and/or software capable of amplifying a signal. Inthe embodiment illustrated in FIG. 2, gain stage 210 is configuredsimilar to gain stage 110 discussed above with respect to FIG. 1. Thatis, gain stage 210 generates an output signal having a substantiallyconstant noise component when gain stage 210 is in a “hold” mode(discussed below).

Passive filtered-sampling stage 250 includes a switch 255 (e.g., asingle pole, single throw switch (SPST)) to selectively couple passivefiltered-sampling stage 250 to gain stage 210. In addition, passivefiltered-sampling stage 250 includes a resistive element 270 (e.g., aresistor) having a resistance in the range of about 10 kΩ to about 100kΩ coupled in series with switch 255 and coupled to a node 265, whereinnode 265 is also connected to the output of capacitive sensor 200.

Passive filtered-sampling stage 250 also includes a capacitive element260 (e.g., a capacitor) coupled to node 265. Capacitive element 260includes a capacitance in the range of about 2 pF to about 20 pF and isconfigured to sample a signal when switch 255 is closed and hold/storethe signal when switch 255 is opened.

In an exemplary operational mode, capacitive sensor 200 is configured tooutput a signal having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component with areduced amount of noise. The following is a description of howcapacitive sensor 200 operates:

Initially, switches 130 and 155 are closed, switch 116 is connected toV_(ref), and switch 118 is connected to ground. This results in theoutput signal being equal to V_(ref).

Switch 130 is opened, while switch 255 remains closed. Shortly afterswitch 130 is opened, switches 116 and 118 are switched so that voltagesteps of V_(ref) and −V_(ref) are applied to capacitive elements 114 and112, respectively.

The change in charge (i.e., C_(s)−C_(ref)) is stored at capacitiveelement 125, which leads to a change in the output signal by the desiredamount of V_(ref)*(C_(s)−C_(ref))/C₁₂₅. Switch 255 is then opened sothat the present value of the output signal is stored on capacitiveelement 260.

The addition of a relatively large resistive element 270 between switch255 and capacitive element 260 forms a low pass filter that attenuatesthe noise created by operational amplifier 120, switch 130, and/orswitch 255. That is, it has been found that capacitive sensor 200 isapproximately an 8 dB improvement over capacitive sensor 100.

FIG. 3 is a schematic diagram of another exemplary embodiment of acapacitive sensor 300 having a gain stage 310 and a passivefiltered-sampling stage 350, wherein gain stage 310 is configuredsimilar to gain stage 210 discussed above with reference to FIG. 2. Asillustrated in FIG. 3, passive filtered-sampling stage 350 includes abranch 360 coupled in parallel with a branch 380.

Branch 360 includes a switch 365 (e.g., a SPST switch) selectivelycoupling branch 360 to gain stage 310. Switch 365 is also coupled to acapacitive element 370 (e.g., a capacitor) having a capacitance in therange of about 2 pF to about 20 pF via a node 375. Capacitive element370 is configured to sample a signal from gain stage 310 when switch 365is closed and to hold/store the signal when switch 365 is opened.

Branch 380 includes a switch 385 (e.g., a SPST switch) selectivelycoupling branch 380 to gain stage 310. Switch 385 is also coupled to acapacitive element 390 (e.g., a capacitor) having a capacitance in therange of about 2 pF to about 20 pF via a node 395. Capacitive element390 is configured to sample a signal from gain stage 310 when switch 385is closed and to hold/store the signal when switch 385 is opened.

Passive filtered-sampling stage 350 also includes a subtractor 398coupled to nodes 375 and 395, and coupled to the output (V_(out)) ofcapacitive sensor 300. As illustrated in FIG. 3, subtractor 398 isconfigured to subtract a signal held on capacitive element 390 from asignal held on capacitive element 370. However, various embodimentscontemplate that subtractor 398 may be configured to subtract a signalheld on capacitive element 370 from a signal held on capacitive element390.

In an exemplary operational mode, capacitive sensor 300 is configured tooutput a signal having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component with areduced amount of noise. The following is a description of howcapacitive sensor 300 operates:

Initially, switches 130, 365, and 385 are each open, switch 116 isconnected to V_(ref), and switch 118 connected to ground. Switch 130 isthen closed to reset the circuit.

Switch 130 is opened and switch 385 is closed, while switch 365 remainsopen. This generates a signal (V₁) from gain stage 310, wherein V₁includes noise from operational amplifier 120 and noise from theswitching action of switch 130. The V₁ signal is then sampled bycapacitive element 390.

Switch 385 is opened so that the V₁ signal is held on capacitive element390, and now the held V₁ signal also includes noise from the switchingaction of switch 385. Shortly thereafter, switches 116 and 118 areswitched so that voltage steps of V_(ref) and −V_(ref) are applied tocapacitive elements 114 and 112, respectively.

Switch 365 is then closed so that gain stage 310 generates a signal (V₂)having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component, noise from operationalamplifier 120, and noise from the switching action of switch 130. The V₂signal is then sampled by capacitive element 370, and switch 365 isopened so that the signal V₂ signal is held on capacitive element 370(which held V₂ signal also includes noise from the switching action ofswitch 365).

Subtractor 398 then subtracts the held V₁ signal from the held V₂signal. Accordingly, the output signal from capacitive sensor 300 hasthe V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component with a reduced amount ofnoise. That is, V₂−V₁→[V_(ref)*(C_(s)−C_(ref))/C₁₂₅ plus operationalamplifier 120 noise, plus switch 130 noise, plus switch 365 noise] minus[operational amplifier 120 noise plus switch 130 noise, plus switch 385noise] equals V_(ref)*(C_(s)−C_(ref))/C₁₂₅ (plus switch 365 noise minusswitch 385 noise). Accordingly, the output signal substantially equalsV_(ref)*(C_(s)−C_(ref))/C₁₂₅, which has been shown to be a 4 dBimprovement over the output of capacitive sensor 100.

Notably, as suggested above, the operation of branches 360 and 380 maybe reversed. That is, passive filtered-sampling stage 350 may beconfigured such that capacitive element 370 holds V₁ and capacitiveelement 390 holds V₂ while subtractor 398 subtracts the signal (V₁) heldon capacitive element 370 from the signal (V₂) held on capacitiveelement 390 to obtain an output signal of V_(ref)*(C_(s)−C_(ref))/C₁₂₅.

FIG. 4 is a schematic diagram of yet another exemplary embodiment of acapacitive sensor 400 having a passive gain stage 410 and a passivefiltered-sampling stage 450, wherein gain stage 410 is configuredsimilar to gain stage 210 discussed above with reference to FIG. 2. Asillustrated in FIG. 4, gain stage 410 includes a branch 460 coupled inparallel with a branch 480.

Branch 460 includes a switch 465 (e.g., a SPST switch) selectivelycoupling branch 460 to gain stage 410. Switch 465 is coupled in serieswith a resistive element 467 (e.g., a resistor) having a resistance inthe range of about 10 kΩ to about 100 kΩ. Resistive element 467 iscoupled to a capacitive element 470 (e.g., a capacitor) having acapacitance in the range of about 2 pF to about 20 pF via a node 475,wherein capacitive element 470 is configured to sample a signal fromgain stage 410 when switch 465 is closed and hold/store the signal whenswitch 465 is opened.

Branch 480 includes a switch 485 (e.g., a SPST switch) selectivelycoupling branch 480 to gain stage 410. Switch 485 is coupled in serieswith a resistive element 487 (e.g., a resistor) having a resistance inthe range of about 10 kΩ to about 100 kΩ. Resistive element 487 iscoupled to a capacitive element 490 (e.g., a capacitor) having acapacitance in the range of about 2 pF to about 20 pF via a node 495,wherein capacitive element 490 is configured to sample a signal fromgain stage 410 when switch 485 is closed and hold/store the signal whenswitch 485 is opened.

Passive filtered-sampling stage 450 also includes a subtractor 498coupled to nodes 475 and 495, and coupled to the output (V_(out)) ofcapacitive sensor 400. As illustrated in FIG. 4, subtractor 498 isconfigured to subtract a signal held on capacitive element 490 from asignal held on capacitive element 470. However, various embodimentscontemplate that subtractor 498 may be configured to subtract a signalheld on capacitive element 470 from a signal held on capacitive element490.

In an exemplary operational mode, capacitive sensor 400 is configured tooutput a signal having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component with areduced amount of noise. To accomplish such, capacitive sensor 400operates as follows:

Initially, switches 130, 465, and 485 are each open, switch 116 isconnected to V_(ref), and switch 118 connected to ground. Switch 130 isthen closed to reset the circuit.

Switch 130 is opened and switch 465 is closed (switch 485 remains open)so that gains stage 410 generates a signal (V₁) including noise fromoperational amplifier 120 and noise from the switching action of switch130. The V₁ signal is sampled by capacitive element 470 and switch 465is opened so that the V₁ signal is held on capacitive element 470,wherein the held V₁ signal also includes noise from the switching actionof switch 465.

Shortly thereafter, switches 116 and 118 are switched so that voltagesteps of V_(ref) and −V_(ref) are applied to capacitive elements 114 and112, respectively. Switch 485 is then closed so that gain stage 410generates a signal (V₂) having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component,noise from operational amplifier 120, and noise from the switchingaction of switch 130.

The V₂ signal is sampled by capacitive element 490 and switch 485 isopened so that the V₂ signal is held on capacitive element 490 (whichheld V₂ signal also includes noise from the switching action of switch485). Subtractor 498 subtracts the held V₁ signal from the held V₂signal so that the output signal from capacitive sensor 400 has theV_(ref)*(C_(s)−C_(ref))/C₁₂₅ component. That is,V₂−V₁→[V_(ref)*(C_(s)−C_(ref))/C₁₂₅ plus operational amplifier 120noise, plus switch 130 noise, plus switch 485 noise] minus [operationalamplifier 120 noise plus switch 130 noise, plus switch 465 noise] equalsV_(ref)*(C_(s)−C_(ref))/C₁₂₅ plus switch 485 noise minus switch 465noise.

The addition of resistive elements 467 and 487 attenuates the noisegenerated by operational amplifier 120, switch 130, switch 465, and/orswitch 485. Accordingly, output signals of capacitive sensor 400substantially equal V_(ref)*(C_(s)−C_(ref))/C₁₂₅, which has been shownto be a 12 dB improvement over the output of capacitive sensor 100.

Notably, as suggested above, the operation of branches 460 and 480 maybe reversed. That is, passive filtered-sampling stage 450 may beconfigured such that capacitive element 490 holds V₁ and capacitiveelement 470 holds V₂ while subtractor 498 subtracts the signal (V₁) heldon capacitive element 490 from the signal (V₂) held on capacitiveelement 470 to obtain an output signal of V_(ref)*(C_(s)−C_(ref))/C₁₂₅.

FIG. 5 is a schematic diagram of one exemplary embodiment of acapacitive sensor 500 having a gain stage 510 and an activefiltered-sampling stage 550, wherein gain stage 510 is configuredsimilar to gain stage 210 discussed above with reference to FIG. 2.Active filtered-sampling stage 550 includes a resistive element 555(e.g., a resistor) having a resistance in the range of about 10 kΩ toabout 100 kΩ coupled to gain stage 510 and to a node 560.

Node 560 is also coupled to a resistive element 565 (e.g., a resistor)having a resistance in the range of about 10 kΩ to about 100 kΩ, whereinresistive elements 565 and 555 may have substantially the same amount ofresistance. Furthermore, resistive element 565 is coupled to a node 570,wherein node 570 is coupled to an output (V_(out)) of capacitive sensor500.

In addition, node 560 is selectively coupled to an integrator circuit580 via a switch 575 (e.g., a SPST switch). Integrator circuit 580includes an operational amplifier 585 (similar to operational amplifier120) having a negative input, a positive input, and an output, and acapacitive element 590 (e.g., a capacitor) coupled to the negative inputand to the output of operational amplifier 585. Additionally, the outputof operational amplifier 585 and capacitive element 590 are each coupledto node 570.

In an exemplary operational mode, capacitive sensor 500 is configured tooutput a signal having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component with areduced amount of noise. To accomplish such, capacitive sensor 500operates as follows:

Initially, switches 130 and 575 are each open, switch 116 is connectedto V_(ref), and switch 118 connected to ground. Switches 130 and 575 arethen closed and shortly thereafter, switches 116 and 118 are switched sothat voltage steps of V_(ref) and −V_(ref) are applied to capacitiveelements 114 and 112, respectively. This results in gain stage 510generating a signal (V₁) having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅component, noise from operational amplifier 120, noise from switch 130,and noise from switch 575 (i.e., V₁=[V_(ref)*(C_(s)−C_(ref))/C₁₂₅] plusoperational amplifier 120 noise, plus switch 130 noise, plus switch 575noise).

The V₁ signal is sampled by capacitive element 590, and switches 130 and575 are each opened so that a signal (V₂) having operational amplifier585 noise is subtracted from the V₁ signal. Accordingly, the outputsignal (V_(out)) of capacitive sensor 500 is V₁−V₂ (i.e.,[V_(ref)*(C_(s)−C_(ref))/C₁₂₅] plus operational amplifier 120 noise,plus switch 130 noise, plus switch 575 noise, minus operationalamplifier 585 noise →V_(out) equals V_(ref)*(C_(s)−C_(ref))/C₁₂₅ plusswitch 130 noise, plus switch 575 noise).

Resistive element 555 and capacitive element 590 form a low pass filterthat attenuates the noise created by operational amplifier 120, switch130, and/or switch 575. That is, it has been found that capacitivesensor 500 is approximately an 8 dB improvement over capacitive sensor100.

FIG. 6 is a schematic diagram of another exemplary embodiment of acapacitive sensor 600 having a gain stage 610 and an activefiltered-sampling stage 650, wherein gain stage 610 is configuredsimilar to gain stage 210 discussed above with reference to FIG. 2.Active filtered-sampling stage 650 includes a resistive element 620(e.g., a resistor) coupled to gain stage 610 via a node 605 and coupledto a node 625. Node 625 is also coupled to a resistive element 630(e.g., a resistor) having a resistance in the range of about 10 kΩ toabout 100 kΩ, wherein resistive element 630 is also coupled to ground.

Furthermore, active filtered-sampling stage 650 includes a switch (e.g.,a SPST switch) selectively coupling node 625 to a node 635. Node 635 isalso coupled to a integrator circuit 680 and a capacitive element 640(e.g., a capacitor) having a capacitance in the range of about 2 pF toabout 20 pF, wherein capacitive element 640 is also coupled to ground.

Integrator circuit 680 includes an operational amplifier 685 having anegative input, a positive input, and an output, and a capacitiveelement 690 (e.g., a capacitor) coupled to the negative input and to theoutput of operational amplifier 685, wherein capacitive elements 640 and690 may have substantially the same amount of capacitance. Moreover, thepositive input of operational amplifier 685 is coupled to node 635.

Active filtered-sampling stage 650 also includes a resistive element 655(e.g., a resistor) having a resistance in the range of about 10 kΩ toabout 100 kΩ coupled to gain stage 610 via node 605 and coupled to anode 660. Node 660 is also coupled to a resistive element 665 having aresistance in the range of about 10 kΩ to about 100 kΩ, whereinresistive elements 665 and 655 have substantially the same amount ofresistance. Furthermore, resistive element 665 is coupled to a node 670,wherein node 670 is coupled to an output of capacitive sensor 600.

In addition, node 660 is selectively coupled to a integrator circuit 680via a switch 675 (e.g., a SPST switch). Additionally, node 670 iscoupled to the output of operational amplifier 685 and to capacitiveelement 690.

In an exemplary operational mode, capacitive sensor 600 is configured tooutput a signal having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component with areduced amount of noise. To accomplish such, capacitive sensor 600operates as follows:

Initially, switches 130, 627, and 675 are each open, switch 116 isconnected to V_(ref), and switch 118 connected to ground. Switch 130 isthen closed to reset the circuit.

Switch 130 is opened and switch 627 is closed (switch 675 remains open)so that gains stage 610 generates a signal (V₁) including noise fromoperational amplifier 120 and noise from the switching action of switch130.

The V₁ signal is sampled by capacitive element 640, and switch 627 isopened so that the signal V₁ is held on capacitive element 640, whichheld V₁ signal also includes noise from the switching action of switch627. Shortly thereafter, switches 116 and 118 are switched so thatvoltage steps of V_(ref) and −V_(ref) are applied to capacitive elements114 and 112, respectively.

Switch 675 is then closed so that gain stage 610 generates a signal (V₂)having a V_(ref)*(C_(s)−C_(ref))/C₁₂₅ component, noise from operationalamplifier 120, and noise from the switching action of switch 130. The V₂signal is sampled by capacitive element 690, and switch 675 is opened sothat the signal V₂ is held on capacitive element 690 (which held V₂signal also includes noise from the switching action of switch 675).

Operational amplifier 685 subtracts V₁ from V₂ so that output signals(V_(out)) from capacitive sensor 600 have theV_(ref)*(C_(s)−C_(ref))/C₁₂₅ component. That is,V₂−V₁→[V_(ref)*(C_(s)−C_(ref))/C₁₂₅ plus operational amplifier 120noise, plus switch 130 noise, plus switch 675 noise] minus [operationalamplifier 120 noise plus switch 130 noise, plus switch 627 noise] equalsV_(ref)*(C_(s)−C_(ref))/C₁₂₅ plus switch 675 noise minus switch 627noise.

Since switches 627 and 675 are similar, the noise generated by theswitching action of these respective switches is consideredsubstantially the same so that the switch 675 noise minus switch 627noise component of the output signals is considered to equal zero.Accordingly, the output signal of capacitive sensor 600 substantiallyequals V_(ref)*(C_(s)−C_(ref))/C₁₂₅, which has been shown to be a 12 dBimprovement over the output of capacitive sensor 100.

FIG. 7 is a flow diagram representing one exemplary embodiment of amethod 700 for reducing noise in a capacitive sensor (e.g., capacitivesensors 200, 300, 400, 500, and 600). Method 700 begins byresetting/clearing the capacitive sensor (block 705) and generating asignal (e.g., S₁) having a noise component (e.g., operational amplifier120 noise, switch 130 noise, etc.) in a gain stage (e.g., gain stages210, 310, 410, 510, and 610) (block 710).

The S₁ signal is sampled by a component (e.g., capacitors 260, 370, 390,470, 490, 590, and 640) of a filtered-sampling stage (e.g.,filtered-sampling stages 250, 350, 450, 550, and 650) (block 715). Thecomponent of the filtered-sampling stage is isolated from the gain stage(block 720) and the S₁ signal is at least temporarily stored in thecomponent of the filtered-sampling stage (block 725).

A signal (e.g., S₂) having a desired output (e.g.,V_(ref)*(C_(s)−C_(ref))/C₁₂₅) component and the noise included in the S₁signal is generated in the gain stage (block 730) and sampled by adifferent component (e.g., capacitors 370, 390, 470, 490, 590, and 640)of the filtered-sampling stage (block 735). The component is isolatedfrom the gain stage (block 740) and the S₂ signal is at leasttemporarily stored in the portion of the filtered-sampling stage (block745).

The S₁ signal is subtracted (e.g., via subtractors 398 and 498, or viaoperational amplifiers 585 and 685) from the S₂ signal so that theoutput signal of the capacitive sensor includes theV_(ref)*(C_(s)−C_(ref))/C₁₂₅ component with a reduce amount of noise(block 750). Since the amount a noise generated by the variouscomponents of the gain stage may change over time, the method defined byblocks 710-750 may be repeated for each output signal of the capacitivesensor (block 755).

In summary, various exemplary embodiments provide a capacitive sensorcomprising a gain stage including an output, the gain stage configuredto generate a first signal having a noise component and generate asecond signal having an output component of the gain stage and the noisecomponent. The capacitive sensor also includes a filtered sampling stagehaving an input coupled to the gain stage output, the filtered-samplingstage configured to sample the first signal, store the first signal, andsubtract the first signal from the second signal. Furthermore, aresistive element of the resistive element-capacitive element, in oneembodiment, includes about 100 kΩ of resistance.

In one embodiment, the filter-sampling stage comprises a passive lowpass filter. In another embodiment, the filtered-sampling stage includesa switch selectively coupling the filtered-sampling stage input to thegain stage output and a resistive element-capacitive element circuitcoupled to the switch.

The filtered-sampling stage, in another exemplary embodiment, includes asubtractor, a first branch having an input and an output coupled to thesubtractor, and a second branch having an input and an output coupled tothe subtractor, the second branch coupled in parallel with a firstbranch. In this embodiment, the first branch includes a first switchselectively coupling the first branch to the gain stage output, and afirst capacitive element coupled to the first switch, the subtractor,and to ground, and the second branch includes a second switchselectively coupling the second branch to the gain stage output, and asecond capacitive element coupled to the second switch, the subtractor,and to ground. In this embodiment, the first capacitive element may beconfigured to sample the first signal when the first switch is closedand also store the first signal after the first switch is opened.Similarly, the second capacitive element may be configured to sample thesecond signal when the second switch is closed and store the secondsignal after the second switch is opened. Furthermore, the subtractormay be configured to subtract the first signal from the second signal.

In another exemplary embodiment, he filtered-sampling stage includes asubtractor, a first branch having an input and an output coupled to thesubtractor, and a second branch having an input and an output coupled tothe subtractor, the second branch coupled in parallel with a firstbranch. In this embodiment, the first branch includes a first switchselectively coupling the first branch to the gain stage output, and afirst resistive element-capacitive element circuit coupled to the firstswitch, the subtractor, and to ground. Similarly, the second branchincludes a second switch selectively coupling the second branch to thegain stage output, and a second resistive element-capacitive elementcircuit coupled to the second switch, the subtractor, and to ground. Inaccordance with one aspect of this embodiment, a resistive element inone of the first resistive element-capacitive element circuit and thesecond resistive element-capacitive element circuit includes about 100kΩ of resistance.

Another exemplary embodiment provides a capacitive sensor including anoutput and having a gain stage including a gain stage output, thecapacitive sensor comprising an active filtered-sampling stage having aninput coupled to the gain stage output. In one aspect of thisembodiment, the active filtered-sampling includes a first resistiveelement having an input coupled to the output of the gain circuit andhaving an output coupled to a first node, a second resistive elementhaving an input coupled to the first node and an output coupled to thecapacitive sensor output, and a first switch selectively coupling thefirst node to a integrator circuit, wherein the integrator circuit iscoupled to the capacitive sensor output.

In one embodiment of the capacitive sensor, the gain stage includes afirst operational amplifier, the integrator circuit includes a secondoperational amplifier having a positive input, a negative input, and anoperational amplifier output, and the integrator circuit includes afirst capacitive element coupled to the negative input and theoperational amplifier output. In accordance with one aspect of thisembodiment, the active filtered-sampling stage further includes a thirdresistive element having an input coupled to the output of the gaincircuit and having an output coupled to a second node, a fourthresistive element having an input coupled to the second node and anoutput coupled to ground, a second switch selectively coupling thesecond node to a third node, and a second capacitive element having aninput coupled to the third node and an output coupled to ground, whereinthe third node is coupled to the positive input of the operationalamplifier. In another aspect, the first resistive element, the secondresistive element, the third resistive element, and the fourth resistiveelement each include substantially the same amount of resistance, andthe first capacitive element and the second capacitive element havesubstantially the same amount of capacitance. In yet another aspect, thefirst operational amplifier and the second operational amplifier eachgenerate substantially the same amount of noise.

A method for reducing noise in a sensor having a gain stage is alsoprovided in various other exemplary embodiments. The method includesgenerating a first signal having a first noise component of the gainstage, storing the first signal, generating a second signal comprisingan output component of the gain stage and the first noise component, andsubtracting the first signal from the second signal. In accordance withone aspect of this embodiment, storing the first signal includestemporarily storing the first signal until the next time the sensor isreset.

In accordance with another exemplary embodiment, the method furtherincludes resetting the sensor, generating a third signal having a secondnoise component different from the first noise component, storing thethird signal, generating a fourth signal comprising the output componentand the second noise component, and subtracting the third signal fromthe fourth signal.

The first signal, in one embodiment, is stored in a first capacitiveelement, the method further comprising isolating the first capacitiveelement from the gain stage prior to generating the second signal. Inanother embodiment, the method further includes storing the secondsignal in a second capacitive element, and isolating the secondcapacitive element, wherein the second signal is stored and the secondcapacitive element is isolated prior to subtracting the first signalfrom the second signal.

In accordance with another exemplary embodiment, generating the secondsignal comprises generating the second signal utilizing a integratorcircuit. The first noise component, in one aspect of this embodiment, isnoise of the integrator circuit. In another aspect, resetting includesisolating the integrator circuit and adding the unity gain noise to thefirst signal.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

1. A capacitive sensor comprising: a gain stage including an output, thegain stage configured to generate a first signal having a noisecomponent and generate a second signal having an output component of thegain stage and the noise component; and a filtered-sampling stage havingan input coupled to the gain stage output, the filtered-sampling stageconfigured to: sample the first signal, store the first signal, andsubtract the first signal from the second signal.
 2. The capacitivesensor of claim 1, wherein the filter-sampling stage comprises a passivelow pass filter.
 3. The capacitive sensor of claim 1, thefiltered-sampling stage comprising: a switch selectively coupling thefiltered-sampling stage input to the gain stage output; and a resistiveelement-capacitive element circuit coupled to the switch.
 4. Thecapacitive sensor of claim 3, wherein a resistive element of theresistive element-capacitive element circuit includes about 100 kΩ ofresistance.
 5. The capacitive sensor of claim 1, the filtered-samplingstage comprising: a subtractor; a first branch having an input and anoutput coupled to the subtractor; and a second branch having an inputand an output coupled to the subtractor, the second branch coupled inparallel with an integrator circuit, wherein the first branch includes:a first switch selectively coupling the first branch to the gain stageoutput, and a first capacitive element coupled to the first switch, thesubtractor, and to ground; and wherein the second branch includes: asecond switch selectively coupling the second branch to the gain stageoutput, and a second capacitive element coupled to the second switch,the subtractor, and to ground.
 6. The capacitive sensor of claim 5,wherein: the first capacitive element is configured to sample the firstsignal when the first switch is closed, and the first capacitive elementis configured to store the first signal after the first switch isopened.
 7. The capacitive sensor of claim 6, wherein: the secondcapacitive element is configured to sample the second signal when thesecond switch is closed, the second capacitive element is configured tostore the second signal after the second switch is opened, and thesubtractor is configured to subtract the first signal from the secondsignal.
 8. The capacitive sensor of claim 1, the filtered-sampling stagecomprising: a subtractor; a first branch having an input and an outputcoupled to the subtractor; and a second branch having an input and anoutput coupled to the subtractor, the second branch coupled in parallelwith an integrator circuit, wherein the first branch includes: a firstswitch selectively coupling the first branch to the gain stage output,and a first resistive element-capacitive element circuit coupled to thefirst switch, the subtractor, and to ground; and wherein the secondbranch includes: a second switch selectively coupling the second branchto the gain stage output, and a second resistive element-capacitiveelement circuit coupled to the second switch, the subtractor, and toground.
 9. The capacitive sensor of claim 8, wherein a resistive elementin one of the first resistive element-capacitive element circuit and thesecond resistive element-capacitive element circuit includes about 100kΩ of resistance.
 10. A method for reducing noise in a sensor having again stage, the method comprising: generating a first signal having afirst noise component of the gain stage; storing the first signal;generating a second signal comprising an output component of the gainstage and the first noise component; and subtracting the first signalfrom the second signal.
 11. The method of claim 10, wherein storing thefirst signal comprises temporarily storing the first signal until thenext time the sensor is reset.
 12. The method of claim 11, furthercomprising: resetting the sensor; generating a third signal having asecond noise component different from the first noise component; storingthe third signal; generating a fourth signal comprising the outputcomponent and the second noise component; and subtracting the thirdsignal from the fourth signal.
 13. The method of claim 10, wherein thefirst signal is stored in a first capacitive element, the method furthercomprising isolating the first capacitive element from the gain stageprior to generating the second signal.
 14. The method of claim 13,further comprising: storing the second signal in a second capacitiveelement; and isolating the second capacitive element, wherein the secondsignal is stored and the second capacitive element is isolated prior tosubtracting the first signal from the second signal.
 15. The method ofclaim 10, wherein generating the second signal comprises generating thesecond signal utilizing an integrator circuit.
 16. The method of claim15, wherein the first noise component is noise of the integratorcircuit, and wherein resetting comprises isolating the integratorcircuit and adding the unity gain noise to the first signal.